Method to tune emission wavelength of wavelength tunable laser apparatus and laser apparatus

ABSTRACT

A method to tune an emission wavelength of a wavelength tunable laser apparatus is disclosed. The laser apparatus implements, in addition to a wavelength tunable laser diode (t-LD) integrating with a semiconductor optical amplifier (SOA), a wavelength monitor including an etalon filter. The current emission wavelength is determined by a ratio of the magnitude of a filtered beam passing the etalon filter to a raw beam not passing the etalon filter. The method first sets the SOA in an absorbing mode to sense stray component disturbing the wavelength monitor, then correct the ratio of the beams by subtracting the contribution from the stray component.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. patent application Ser. No.14/830,585 filed Aug. 19, 2015, which claims the benefit of Japan PatentApplication No. 2014-168747, filed Aug. 21, 2014.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present application relates to a laser system, in particular,relates to a wavelength tunable laser system.

2. Background Arts

A laser module that implements a wavelength tunable laser diode (t-LD)accompanies with a wavelength detector, or a wavelength locker to tunean emission wavelength of the t-LD. A t-LD often accompanies with asemiconductor optical amplifier (SOA) to amplify light generated in anLD having a wavelength tunable function, because the light output fromthe LD with the wavelength tunable function is often limited in outputpower thereof The SOA is usually integrated with the LD with thewavelength tunable function.

The wavelength detector, or the wavelength locker, implements an opticalcomponent inherently showing a specific transmittance, typically anetalon filter. Detecting a magnitude of raw light not passing the etalonfilter and that of filtered light passing through the etalon filter, andcomparing a ratio of the magnitude of the filtered light to that of theraw light with the transmittance of the etalon filter, the wavelength ofthe raw light, namely, the wavelength of the light currently emittedfrom the LD may be estimated. The wavelength tunable module may tune theemission wavelength thereof so as to coincide the current emissionwavelength thus estimated with the target wavelength.

Recent optical communication systems request five or more figures in thepreciseness of the signal wavelength. An optical module implementing at-LD therein applicable to such communication system inevitablyimplements the superior exactness of the emission wavelength by acompact arrangement and simplified procedures.

SUMMARY OF THE INVENTION

An aspect of the present application relates to a method to tune anemission wavelength of a laser apparatus, where the laser apparatusincludes (a) a wavelength tunable laser diode (t-LD) that integrates alaser diode (LD) with a semiconductor optical amplifier (SOA), and (b) awavelength monitor that senses the emission wavelength of the t-LD. Inthe laser apparatus of the present application, the wavelength monitoris disposed in one of a front side and a rear side of the t-LD, andincludes an optical filter, a first photodiode (PD) that senses a rawbeam, which is not transmitted through the optical filter, and a secondPD that senses a filtered beam, which is split from the raw beam andtransmitted through the optical filter. The method of the presentapplication comprises steps of: (1) evaluating a first stray componentand a second stray component by a first PD and a second PD,respectively, where the first stray component and the second straycomponent originate to an optical beam output from another of the frontside and the rear side not disposing the wavelength monitor; (2) sensingthe raw beam and the filtered beam by the first PD and the second PD,respectively; and (3) calculating a ratio of the filtered beamsubtracted with the second stray component to the raw beam subtractedwith the first stray component.

Another aspect of the present application relates to a laser apparatusthat comprises (a) a wavelength tunable laser diode (t-LD) thatintegrates a semiconductor laser diode (LD) with a semiconductor opticalamplifier (SOA), where the t-LD emits an optical beam through the SOAfrom a facet thereof and another optical beam from another facetopposite to the facet; (b) a wavelength monitor that includes an opticalfilter, a first photodiode (PD), which senses a raw beam nottransmitting through the optical filter, a second PD, which senses afiltered beam split from the raw beam and transmitting through theoptical filter, where the raw beam and the filtered beam originate toone of the optical beam and the another optical beam; and (c) acontroller that calculates a ratio of the filtered beam subtracted witha second stray component to the raw beam subtracted with a first straycomponent. In the laser apparatus, the first stray component and thesecond stray component are not originated to the wavelength monitor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a functional block diagram of a laser apparatus according tothe first embodiment of the present application;

FIG. 2 is a cross section of the wavelength tunable LD implemented withthe laser apparatus shown in FIG. 1, which is taken along the line II-IIindicated in FIG. 1;

FIG. 3 is a plan view of a laser module implementing the laser apparatusshown in FIG. 1;

FIG. 4 is a flowchart to estimate the current emission wavelength of at-LD implemented within the laser apparatus of FIG. 1;

FIG. 5 is a flowchart to estimate the current emission wavelength takenthe factor of the stray component into accounts;

FIG. 6A shows a mechanism to remove the contribution of the straycomponent according to the first embodiment, FIG. 6B shows a mechanismto remove the contribution of the stray component according to thesecond embodiment, and FIG. 6C shows a mechanism to remove thecontribution of the stray component in the third embodiment;

FIG. 7 is a functional block diagram of a laser apparatus according tothe third embodiment; and

FIG. 8 is a functional block diagram of a laser apparatus according tothe fourth embodiment.

DESCRIPTION OF EMBODIMENTS

Next, some embodiments according to the present application will bedescribed as referring to drawings. In the description of the drawings,numerals or symbols same with or similar to each other will refer toelements same with or similar to each other without duplicatedexplanations.

First Embodiment

FIG. 1 schematically illustrates a functional block diagram of awavelength tunable system (hereafter simply denoted as laser system) 1according to the first embodiment of the present application. The lasersystem 1 includes a light-generating device 2 integrating a laser diode(hereafter simply denoted as LD) 3 with a semiconductor opticalamplifier (hereafter simply denoted as SOA) 4, optical splitters(hereafter simply denoted as first and second BSs), 5A and 5B, a firstoptical detector 7A, an etalon filter 8, a second optical detector 7B,and a controller 10 integrating with a memory 11. The LD 3 is served asa wavelength tunable semiconductor laser diode, and thus thelight-generating device 2 is also served as a wavelength tunablesemiconductor laser diode. Therefore, the light-generating device issimply denoted as t-LD hereafter.

FIG. 2 shows a cross section of the t-LD 2 taken along the line II-IIindicated in FIG. 1. As described, the t-LD 2 monolithically integratesthe LD 3 with the SOA 4 on a semiconductor substrate 21. Thesemiconductor substrate 21 may be made of InP and provides a backelectrode 22 in a whole surface thereof The back electrode 22 is commonto the LD 3 and the SOA 4.

The LD 3 emits light attributed to a wavelength thereof tuned by thecontroller 10. The LD 3 comprises a CSG-DBR (Chirped Sampled GratingDistributed Reflector) region 12 and SG-DFB (Sampled Grating DistributedFeedback) region 13, where the latter region is next to the SOA 4. Thatis, the SG-DFB region 13 is disposed between the SOA 4 and the CSG-DBRregion 12 along the optical axis of the device 2. The SG-DFB region 13provides in a side opposite to the CSG-DBR region 12 a facet 3 a whichoptically couples with the SOA 4, while, the CSG-DBR region 12 providesin a side opposite to the SG-DFB region 13 another facet 3 b.

The CSG-DBR region 12, which is inherently attributed to a reflectionspectrum including a plurality of peaks with a constant span, mayinclude a lower cladding layer 23 made of InP, a waveguide layer 24having a quantum well formed by InGaAsP and so on, an upper claddinglayer 25 made of InP, an insulating film 26 including silicon oxide(SiOx), and a plurality of heaters 27. The waveguide layer 24 isunnecessary to show an optical gain. The heaters 27, which accompanywith respective electrodes, 28 and 29, may adjust temperature of thewaveguide layer 24 along the optical axis, which means that thereflection spectrum attributed to the CSG-DBR region 12, in particular,the wavelength of respective reflection peaks may be varied.Accordingly, a reflection band width may be widened and the emissionwavelength of the LD 3 may be continuously varied.

The lower cladding layer 23 intermittently includes a plurality gratings30 with spaces 31 therebetween, but lengths of the spaces 31 are notuniform. A space 31 a has a length different from lengths of otherspaces 31 b. Such gratings 30 and spaces 31 are called as the chirpedsampled grating and show a convex envelope of the reflection peaks.

The SG-DFB region 13 is inherently attributed with an optical gainspectrum having a plurality of optical gain peaks. The LD 3 may emitlight with the emission wavelength at which one of the reflection peaksof the CSG-DBR region 12 coincides with one of the gain peaks of theSG-DFB region 13. The SG-DFB region 13 includes the lower cladding layer23, which is common to the lower cladding layer 23 in the CSG-DBR region12, an active layer 32 having a quantum well structure including InGaAsPand so on, an upper cladding layer 25, which is also common to the uppercladding layer 25 of the CSG-DBR region 12, a contact layer 33, and anelectrode 34. Injecting carriers into the active layer 32 from theelectrode 34, the SG-DFB region 13 shows the optical gain greater thanunity for causing the laser oscillation in the LD 3.

The lower cladding layer 23 of the SG-DFB region 13 also provides aplurality of gratings 35 with spaces 36 therebetween. Different from theCSG-DBR region 12, the spaces 36 of the SG-DFB region 13 have a lengthcommon to all spaces 36. For instance, the spaces 36 in the SG-DFBregion 13 may be equal to the length of the space 31 b in the CSG-DBRregion 12. The gratings in the CSG-DBR region 12 and those in the SG-DFBregion 13 have a pitch between corrugation same with others.

The SOA 4 includes a lower cladding layer 23, an active layer 37 with aquantum well structure made of InGaAsP and so on, an upper claddinglayer 25, a contact layer 38 made of InGaAsP and so on, and an electrode39. Injecting carriers into the active layer 37 through the electrode39, the SOA 4 may show a function of amplifying light entering thereinfrom the SG-DFB region 13. Adjusting the bias applied to the SOA 4, theSOA 4 shows a function of absorbing light to vanish the optical poweroutput therefrom. Two facets, 4 a and 4 b, exactly the facet 4 b is aninterface against the SG-DFB region 13, face to each other along theoptical axis of the t-LD 2.

Two facets, 2 a and 2 b, of the t-LD 2 provide anti-reflection (AR)films 14 and 15, which show the reflectivity less than 1%. Accordingly,the facets, 2 a and 2 b, may emit light, L₁ and L₂, as shown in FIG. 1.The former light L₁, which is amplified in the SOA 4, becomes the outputlight of the laser system 1. On the other hand, the other light L₂ hasintensity far less than that of the front light L₁ because no SOAs areprovided between the CSG-DBR region 12 and the facet 2 b.

Referring back to FIG. 1, the first BS 5A splits the optical beam L₁coming from the t-LD 2 into two beams, L₃ and L₄. The former beam L₃ isoutput external from the laser apparatus 1, while, the latter beam L₄enters the second BS 5B. The second BS 5B also splits the entering beamL₄ into two beams, L₅ and L₆, where the former beam L₅ enters, or issensed by the first optical detector 7A, and the latter beam is sensedby the second optical detector 7B after passing the etalon filter 8.Because the first and second BSs, 5A and 5B, have the transmittance andthe reflectance substantially independent of wavelengths subject to thet-LD 2 and no functions to convert a wavelength of light, the beam L₅sensed by the first optical detector 7A has the wavelength independentmagnitude and the wavelength thereof is same as the beam L₃ just outputfrom the t-LD 2.

The first and second optical detectors, 7A and 7B, may be typicallyphotodiodes (PD). The first PD 7A, as described above, receives theoptical beam L₅ spilt by the second BS 5B but not passing the etalonfilter 8, while, the second PD 7B receives the optical beam L₇ passingthe etalon filter 8. The etalon filter 8 is an optical filter having aspecific transmittance strongly depending on the wavelength of lightpassing therethrough. Thus, the optical system including the first andsecond PDs, 7A and 7B, and the etalon filter 8 may determine thewavelength of the light just output from the t-LD 2, because the ratioof outputs of the first and second PDs, 7A and 7B, equivalently givesthe transmittance of the etalon filter 8. Comparing thus evaluated ratiowith the transmittance of the etalon filter 8, which is inherently givenby the specification thereof, the wavelength of the optical beam L₇ justpassing the etalon filter 8, corresponds to the emission wavelength ofthe t-LD 2. Accordingly, the optical system including the optical filter8, and the first and second PDs, 7A and 7B, is often called as awavelength monitor, or, as a wavelength locker because the emissionwavelength of the t-LD 2 may be locked at a desired wavelength based onthe monitored wavelength.

The first and second PDs, 7A and 7B, may also sense other light exceptfor the optical beams, L₅ and L₇, respectively. That is, the laserapparatus 1 causes an optical beam L₂ output from the rear facet 2 b ofthe t-LD 2, and those output derived from the optical beams, L₁, and L₃to L₇, but reflected at somewhere toward the first and second PDs, 7Aand 7B, which are called as stray component L₈. The existence of thestray component L₈ affects the evaluation of the ratio of the first andsecond PDs, 7A and 7B. Resultantly, the wavelength determined by theratio becomes incorrect, and the laser apparatus 1 outputs the opticalbeam with an erroneous wavelength.

The controller 10 may be, for instance, a micro-controller including acentral processing unit (CPU) to set the SOA 4 in an absorbing mode orin an amplifying mode by supplying a bias current to the SOA 4. When nobias current is supplied to the SOA 4, the SOA 4 becomes the absorbingmode. On the other hand, the SOA is set in the amplifying mode when thesubstantial bias is supplied.

Also, the controller 10 tunes the wavelength of the optical beam L₁output from the t-LD 2. As described, the outputs of the first andsecond PDs, 7A and 7B, are provided to the controller 10 to evaluate theratio thereof In addition, the controller 10 may evaluate the straycomponents L₈ also detected by the first and second PDs, 7A and 7B.Specifically, the controller 10 sets the SOA 4 in the absorbing mode,which means substantially no optical beam L₁ is output from the t-LD 2,but the optical beam L₂ output from the rear facet 2 b of the t-LD 2 isstill left, which possibly becomes the stray component L₈; then, thecontroller 10 may evaluate the stray components through the outputs ofthe first and second PDs, 7A and 7B. Details of the evaluation of thestray component L₈ will be described later.

The controller 10 may tune the emission wavelength of the t-LD 2comparing thus determined wavelength of the optical beam L₁ currentlyoutput from the t-LD 2 with the target wavelength. Based on a differencebetween the current emission wavelength and the target wavelength, thecontroller 10 may adjust powers supplied to the heaters 27 and biases tothe SG-DFB region 13 so as to coincide the current wavelength with thetarget wavelength. The memory 11 may store the magnitudes of the straycomponent L₈ detected by the first and second PDs, 7A and 7B, wherethese magnitudes are correction values to calculate the ratio of theoutputs of the first and second PDs, 7A and 7B.

FIG. 3 is a plan view showing a laser module 50 implementing the laserapparatus 1 thus described. As illustrated in FIG. 3, the laser module50 provides a housing 51 into which the laser apparatus 1 is installed.The laser apparatus 1 provides a collimating lens 52 between the t-LD 2and the first BS 5A, which collimates the optical beam L₁ output fromthe front facet 2 a of the t-LD 2. The t-LD 2 and the collimating lens52 are set on a thermos-electric cooler (TEC) 53 to stabilize theemission wavelength of the t-LD 2. The optical beam L₁ entering thefirst BS 5A is processed according to the optical system described inFIG. 1 and output from the window 55 of the housing 51 as the formerbeam L₃. The optical system, namely, the first and second BSs, 5A and5B, the etalon filter 8, and the second PD 7B are mounted on another TEC54 that stabilizes a temperature of optical components mounted thereon,in particular, that of the etalon filter 8. Two sides of the housing 51provide lead pins to supply powers to respective TECs, 53 and 54, and torespective heaters 27 of the t-LD 2, and provide biases to the t-LD 2.

As described in FIG. 1, the optical beam L₂ output from the rear facetof the t-LD 2 possibly becomes a stray component L₈ and may berespectively sensed by the first and second PDs, 7A and 7B, afterreflected at somewhere within the housing 51. The magnitudes of thestray component L₈ at the first and second PDs, 7A and 7B, areindeterminable or uncontrollable. Surface conditions of the opticalcomponents, those of the inner wall of the housing 51, relativepositions between the components, and so on all affects the magnitude ofthe stray component. Accordingly, the magnitude of the stray componentis necessary to be sensed in respective laser modules.

Next, one of procedures, or algorithms to tune the emission wavelengthof the laser module 50 will be described as referring to FIGS. 4 and 5.FIG. 4 shows a flow chart to tune the emission wavelength of the lasermodule 50 according to the first embodiment of the present application,where the tuning of the emission wavelength in the target or closelyaround the target wavelength is carried out as ceasing the opticaloutput externally, which is often called as the dark tuning.

First, the process sets the SOA 4 in the absorbing mode by providing anegative bias (the negative bias includes no bias) thereto, at step S1,which forces the output power of the front beam L₁ to be substantiallyzero, or at least far less than the power of the rear beam L₂. Then, thebiases and powers corresponding to the target wavelength are provided tothe LD 3, and the temperature of the t-LD 2 is set constant in the onecorresponding to the target wavelength, at step S2. The t-LD 2 may emitlight by being provided with the biases and powers, but the front beamto be output from the front facet 2 a is substantially absorbed in theSOA 4. Only the rear beam L₂ is practically output from the rear facet 2b of the t-LD 2. The first and second PDs, 7A and 7B, detect the straycomponent L₈ originated to the rear beam L₂, at step S3. FIG. 6A showsthe stray component L₈ at step S3. The outputs of the first and secondPDs, 7A and 7B, are not usually equal to each other reflecting thephysical positions thereof, and the surface conditions of opticalcomponents. The magnitudes of the stray component L₈ thus detected arestored within the memory 11 as correction values, at step S4.

Then, the SOA 4 is set in the amplifying mode by providing a positivebias thereto at step S5. The t-LD 2 outputs the front beam L₁ inaddition to the rear beam L₂. The first and second PDs, 7A and 7B, sensethe raw beam L₅ not passing the etalon filter 8 and the filtered beam L₇passing through the etalon filter 8, respectively. The controller 10receives thus detected magnitudes of the raw beam L₅ and the filteredbeam L₇, at step S6. The controller 10 estimates the current emissionwavelength taking the stored magnitude of the stray component L₈ and themagnitudes of the raw beam L₁ and the filtered beam L₇ into account, atstep S7.

The estimation of the current emission wavelength obeys the flowchartshown in FIG. 5. First, the controller 10 acquires the magnitude of theraw beam L₅ from the first PD 7A, and that of the filtered beam L₇ fromthe second PD 7B, at step S11. Then the controller 10 fetches themagnitudes of the stray component L₈ from the memory 11, at step S12.The controller 10 subtracts the magnitude of the stray component L₈detected by the first PD 7A from that of the raw beam L₅, and that ofanother stray component L₈ detected by the second PD 7B from themagnitude of the filtered beam L₇, at step S13. Thus, two magnitudes ofthe stray component L₈ have a role of the correction factors for the rawbeam L₅ and the filtered beam L₇. Finally, the controller 10 estimatesthe current emission wavelength by a ratio of the corrected magnitude ofthe filtered beam L₇ to the corrected magnitude of the raw beam L₅.Assuming that the magnitude of the stray component L₈ detected by thefirst PD 7A, that by the second PD 7B, that of the raw beam L₅, and thatof the filtered beam L₇, are D1, D2, D3, and D4, respectively, thecontroller 10 may estimate the current emission wavelength by the ratioof (D4-D2)/(D3-D1).

Referring back to FIG. 4, the controller 10 compares the currentemission wavelength thus estimated with the target wavelength at stepS8. When the current emission wavelength is out of a preset range aroundthe target wavelength, which corresponds to “No” in step S8, thecontroller 10 adjusts the biases and powers supplied to the LD 3 so asto set the current emission wavelength closer to the target wavelength,at step S9. Iterating the steps S6 to S9, the current emissionwavelength enters the preset range around the target wavelength, thatis, the current emission wavelength substantially coincides with thetarget wavelength.

A conventional laser apparatus does not correct the factor of the straycomponent in the estimation of the current emission wavelength from theratio of the magnitudes of two PDs. Accordingly, the estimated emissionwavelength does not exactly reflect the current emission wavelength. Onthe other hand, the laser apparatus 1 or the laser module 50 of thepresent application can eliminate the factor of the stray component inthe estimation of the current emission wavelength. The magnitude of theraw beam L₅ and that of the filtered beam L₇ are subtracted with themagnitudes of respective stray component, which are taken in advance tothe estimation of the emission wavelength. Thus, the laser apparatus 1,or the laser module 50, may enhance the accuracy of the estimation ofthe current emission wavelength, which resultantly increases theexactness of the coincidence the current emission wavelength with thetarget wavelength.

The magnitudes of the stray component L₈ originating to the rear beam L₂may be detected under a state where the SOA 4 is set in the absorbingmode, and biases and powers are provided to LD 3 to practically activatethe LD 3, namely, the rear beam L₂ is practically output from the rearfacet 2 b. Although the description above first sets the SOA 4 in theabsorbing mode and then provides the biases and powers to the LD 2; theorder of these two procedures is unconcerned.

The evaluation of the magnitude of the stray component L₈ is necessaryto be carried out before that practical tune of the emission wavelength,but the laser module 50 is unnecessary to evaluate the power of thestray component L₈ in every tunes. When the evaluation of the straycomponent is performed during the delivery inspection of the lasermodule 50 and the magnitudes of the stray component L₈ sensed by thefirst and second PDs, 7A and 7B, are stored in the memory 11, thecontroller 10 may read those magnitudes from the memory 11 and calculatethe accurate wavelength currently emitted from the t-LD 2 during everytuning, which may shorten the time to converge the current emissionwavelength on the target wavelength.

Second Embodiment

The algorithm to estimate the current emission wavelength describedabove first detects the magnitude of the stray component L₈ by settingthe SOA 4 in the absorbing mode. That is, only the rear beam L₂ isoutput from the LD 2 and the first and second PDs, 7A and 7B, sense thestray component L₈ originated to the rear beam L₂. However, the rearbeam L₂ is generally less than the front beam L₁ even when the SOA 4 isset in the amplifying mode. Moreover, the stray component is somehowdetected by the first and second PDs, 7A and 7B, after being reflectedsomewhere several times, and thus the magnitude of the stray componentbecomes faint and strongly affected by noises. When the controller 10processes the magnitude of the detected beam digitally, theanalog-to-digital conversion of the stray component L₈ is affected bythe quantizing error.

The algorithm according to the second embodiment avoids suchdisadvantages described above and has a feature that the stray componentin the magnitude thereof is indirectly sensed. Specifically, referringto FIG. 6B, the SOA 4 is set in the amplifying mode by providing thebias currents in two levels, I_(SAO1) and I_(SOA2), different from eachother. The first and second PDs, 7A and 7B, respectively sense the rawbeam L₅ and the filtered beam L₇ for respective levels. The magnitude ofthe stray component (or namely the magnitude when no front beam L1 beingoutput) can be obtained by extrapolating a plotted line connecting twosensed magnitudes for corresponding bias currents to a point showing thestate of having no bias current. Because the magnitudes, P₁ and P₂, ofthe front beam L₁ are substantial, the first and second PDs, 7A and 7B,may generate respective outputs with substantial intensities alleviatingthe influence from noises and the quantization error when theintensities are processed digitally.

Three or more levels of the bias current are preferable to enhance theaccuracy of the magnitude of the stray component. Moreover, the levelsof the bias current may be selected in a region where the SOA 4 shows alinear dependence in the optical gain thereof against the bias current.An SOA inherently saturates the optical gain thereof in a relativelylarger bias current. Thus, the levels of the bias current may be set ina moderate range.

Third Embodiment

The laser apparatus thus described disposes the wavelength monitor, orthe wavelength locker, in the front side of the t-LD 2. However, a laserapparatus may set the wavelength monitor in the rear side of the t-LD 2.In such an arrangement, the wavelength monitor senses the rear light L₂for evaluating the current emission wavelength. FIG. 7 is a schematicblock diagram of a laser apparatus 1A having the arrangement of the rearmonitor. The laser apparatus 1A provides the second BS 5B, the etalonfilter 8, and the first and second PDs, 7A and 7B, in the rear side ofthe t-LD 2 to sense the rear beam L₂. In the front side of the t-LD 2,the laser apparatus 1A provides the first BS 5A and the third PD 7C tomonitor the magnitude of the front beam L₁.

In the rear monitor arrangement, the first and second PDs, 7A and 7B,are influenced by stray component L₈ originated to the front beam L₁ andthe rear beam L₂. That is, a portion of the front beam L₁ may enter thefirst and second PDs, 7A and 7B, after being reflected several timeswithin the laser module 50. Also, the rear beam L₂ enters the first andsecond PDs, 7A and 7B, through passes except for those of the raw beamL₅ and the filtered beam L₇, after being reflected several times withinthe laser module 50. However, the front beam L₁ has the magnitude fargreater than that of the rear beam L₂; accordingly, the stray componentoriginated to the rear beam L₂ may be ignorable.

In order to evaluate the magnitude of the stray component originated tothe front beam L₁, the algorithm shown in FIG. 6C is applicable. Thatis, the procedure first sets the SOA 4 in the absorbing mode bysupplying no bias current thereto, and fixes the biases and powerssupplied to the LD 3. Under the bias condition for the LD 3 and the SOA4, the front beam L₁ substantially vanish but only the rear beam L₂ isleft, which is referred as the intrinsic rear beam. The magnitude of theraw beam L₅ and the filtered beam L₇ are detected, which will be calledas the intrinsic magnitudes.

Then, setting the bias current supplied to the SOA 4 at an optionalvalue where the SOA 4 is set in the amplifying mode. Under such acondition, a magnitude of the raw beam L₅ and a magnitude of thefiltered beam L₇ are measured. Each of the magnitude of the raw beam L₅and the magnitude of the filtered beam L₇ includes the magnitude of thestray component due to the front beam L₁, and the intrinsic rear beamL₂. Subtracting the intrinsic magnitude from the practically measuredmagnitudes, the magnitude of the stray component due to the front beamL₁ affected to the first and second PDs, 7A and 7B, can be determined asbeing a function of the bias current for the SOA 4. The memory 11 maystore the relation of the stray components L₈ for the first and secondPDs, 7A and 7B, against the bias current. In the practical operation ofthe laser apparatus 1, the bias current for the SOA 4 is optionallyvaried. However, the controller 10 may subtract the stray components L₈stored in the memory 11 from the raw beam and the filtered beam. Thecalculated ratio may be enhanced in the accuracy thereof.

Fourth Embodiment

FIG. 8 shows a block diagram of a laser apparatus 1B according to thefourth embodiment of the present application. The laser apparatus 1B hasa feature distinguishable from the aforementioned laser apparatuses, 1and 1A, that the t-LD 2 provides two SOAs, namely, the front SOA 4F andthe rear SOA 4R, continuous to respective facets, 3 a and 3 b, of the LD3. The front SOA 4F operates as those of the SOA 4 to amplify an opticalbeam generated in the LD 3 and output in forward as the front beam L₁.On the other hand, the rear SOA 4R amplifies an optical beam output fromthe LD 3 rearward or the rear beam L₂ may substantially vanish. Althoughthe block diagram of FIG. 8 does not explicitly illustrate that the rearbeam L₂ is guided outside of the laser apparatus 1B, the rear beam L₂may be utilized as an output beam similar to the front beam L₃.

The first and second PDs, 7A and 7B, to determines the current emissionwavelength of the t-LD 2 are influenced by the stray component L₈originated from both of the front beam L₁ and the rear beam L₂, becausethe rear beam L₂ in the magnitude thereof becomes substantial differentfrom the aforementioned embodiment. That is, the first and second PDs,7A and 7B, may sense not only the raw beam L₅ and the filtered beam L₇,respectively, originated to the rear beam L₂, but the stray component L₈originated to the frond beam L₁ and the rear beam L₂ after beingrandomly reflected several times within the laser module 50.

The estimation of the magnitude of the stray component originated to thefront beam L₁ may be carried out as those similar to the thirdembodiment. That is, setting the rear SOA 4R in the absorbing mode,which means that, in the third embodiment, the rear beam L₂ in themagnitude thereof becomes substantially vanish, and the sensed magnitudeby the first and second PDs, 7A and 7B, directly reflects the magnitudeof the stray component originated to the front beam L₁. On the otherhand, the contribution of the rear beam L₂ to the stray component sensedby the first and second PDs, 7A and 7B, is necessary to isolate thefirst and second PDs, 7A and 7B, from the raw beam L₅ and the filteredbeam L₇, respectively. For instance, setting optical absorbers inrespective outputs of the second BS 5B and the etalon filter 8 so as notto sense the raw beam L₅ and the filtered beam L₇ by the first andsecond PDs, 7A and 7B, the stray component L₈ in the magnitude thereofmay be estimated as a function of the magnitude of the rear beam L₂.

The ratio of the filtered beam L₇ to the raw beam L₅ in the magnitudethereof may be accurately calculated by subtracting the magnitude of thestray component due to the front beam L₁ at the bias current I_(SOA-F)and that due to the rear beam L₂ at the bias current I_(SOA-R) fromrespective magnitudes practically sensed by the first and second PDs, 7Aand 7B. Thus, the current emission wavelength derived from the ratiothus calculated may be accurately determined. Because the set of theoptical absorbers are practically unable to perform in the field afterthe shipment of the laser module 50, the estimation of the magnitude ofthe stray component due to the rear beam L₂, and also by the front beamL₁, is necessary to be performed during the delivery inspection andstored in the memory as the function of the bias currents to the frontSOA 4F and the rear SOA 4R, respectively. In the field, the controller10 may read the magnitude of the stray component L₈ corresponding torespective bias currents from the memory and enhance the accuracy of thecurrent emission wavelength of the laser module 50.

Although the present invention has been fully described in conjunctionwith the preferred embodiment thereof with reference to the accompanyingdrawings, it is to be understood that various changes and modificationsmay be apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims, unless they departtherefrom.

1. A method to tune an emission wavelength of a laser apparatus thatincludes a wavelength tunable laser diode (t-LD) integrating a laserdiode (LD) with a semiconductor optical amplifier (SOA), and awavelength monitor to sense the emission wavelength of the t-LD, thewavelength monitor disposed in one of a front side and a rear side ofthe t-LD, the wavelength monitor including an optical filter, a firstphotodiode (PD) to sense a raw beam not transmitting through the opticalfilter, and a second PD to sense a filtered beam split from the raw beamand transmitting through the optical filter, the method comprising stepsof: evaluating a first stray component and a second stray component bythe first PD and the second PD, respectively, the first stray componentand the second stray component originating to an optical beam outputfrom another of the front side and the rear side not disposing thewavelength monitor; sensing the raw beam and the filtered beam by thefirst PD and the second PD, respectively; and calculating a ratio of thefiltered beam subtracted with the second stray component to the raw beamsubtracted with the first stray component, wherein the SOA is integratedin the front side of the t-LD and the wavelength monitor is disposed inthe front side of the t-LD, and wherein the step of evaluating the firststray component and the second stray component includes steps of:setting the SOA in an absorbing mode; activating the t-LD; and sensingthe first stray component and the second stray component by the first PDand the second PD, respectively. 2-5. (canceled)
 6. The method of claim1, further comprising a step of storing the first stray component andthe second stray component temporarily in a memory before the step ofsensing the raw beam and the filtered beam.
 7. The method of claim 6,further comprising a step of fetching the first stray component and thesecond stray component from the memory before the step of calculatingthe ratio.
 8. The method of claim 6, wherein the step of evaluating andstoring the first stray component and the second stray component arecarried out in a step of a delivery inspection. 9-14. (canceled)
 15. Themethod of claim 6, further comprising steps of: obtaining a firstmagnitude of the raw beam and a second magnitude of the filtered beam inthe step of setting the SOA in the absorbing mode, respectively, settingthe SOA in an amplifying mode after storing the first stray componentand the second stray component temporarily in the memory; respectivelyobtaining a third magnitude of the raw beam and a fourth magnitude ofthe filtered beam by sensing the raw beam and the filtered beam in thestep of setting the SOA in the amplifying mode; and subtracting thefirst magnitude from the third magnitude to evaluating the first straycomponent, and the second magnitude from the fourth magnitude toevaluating the second stray component.
 16. The method of claim 15,further comprising steps of: estimating a current emission wavelength bya ratio of a fifth magnitude to a sixth magnitude; and adjusting thecurrent emission wavelength to a target emission wavelength, wherein thefifth magnitude corresponds to a magnitude obtained by subtracting thefirst magnitude from the third magnitude, and wherein the sixthmagnitude corresponds to a magnitude obtained by subtracting the secondmagnitude from the fourth magnitude.